1. Field of the Invention
The invention relates to high temperature-resistant, electrically conductive thin films, for example in the form of metal structures for producing bondings of piezoelectric components, method for their production as well as elements that comprise the thin film according to the invention.
2. Description of the Related Art
A number of technical applications require electrically conductive metal structures with continuous operating temperatures of 300° C. and more, for example as bonding elements or resistors. While this can be reliably implemented in general in the form of films of thermally resistant metals or electrically conductive compounds with film thicknesses of ≧10 μm, such thick films, in particular in the microstructure and nanostructure range, often can be used only to a limited extent.
Corresponding applications of highly technical, as well as commercial, relevance are, for example, thin-film resistors for measuring physical or chemical parameters, or else piezoelectric systems. Thus, diverse piezo materials are known and are also partially available commercially, which are stable up to 1000° C. and above and contain the piezoelectric properties thereof. At such high temperatures, however, the electrodes that are necessary for the electromechanical coupling represent a significant technical problem as soon as—for technical or other reasons—the usual thick-film metal structures cannot be used. Here, a solution that has been technically satisfactory and commercially implementable to date is lacking, which represents a significant problem, preventing the practical applicability in particular for components based on the principle of surface acoustic waves (AOFW, English surface acoustic waves, SAW).
AOFW elements have proven their value in, e.g., frequency filters as well as in sensors for measuring physical (temperature, pressure, etc.) and/or chemical parameters. Moreover, different applications are known, in which AOFW elements, by themselves or in combination with sensor functionality, are used as radio-readable identification tags (RFID tags). Significant advantages of this technology are that it is possible to remotely query AOFW systems completely passively and that they can be reliably used in principle over a very broad temperature range. Thus, such sensors or RFID systems have significant technological as well as practical advantages relative to conventional semiconductor-based systems. From the literature, low-temperature applications at −200° C. are also known, as are high-temperature applications at up to 1000° C. Corresponding applications comprise, for example, the object tracking of slag pots, or gas sensors, or the temperature monitoring of safety-relevant ceramic components in metallurgy.
Surface structures, in particular interdigital converters and reflectors, applied to the surface of the piezoelectric substrate, are an essential component in the design and creation of AOFW elements. The films that are used for this purpose usually have film thicknesses of ≦1 μm, typically 100-500 nm. Such small film thicknesses allow the controlled generation of complete, as well as partial, reflections of surface waves on surface structures, with which complex structures, such as, for example, reflective delay lines or AOFW resonators, can be produced first; the resulting AOFW elements can be used especially advantageously in succession in sensors and/or RFID tags. Moreover, a small film thickness makes possible an efficient electro-acoustic signal coupling and minimizes the mechanical attenuation of the surface waves in the case of interaction with the structures. In this case, a decisive factor is the surface load, by which the use of the lightest films possible, i.e., of materials of low density, has become important.
Another key criterion in particular for AOFW applications is the possibility of structuring thin-film metal structures precisely with structural widths in the micron to sub-micron range, for example finger-shaped. The necessary structural widths in this case depend essentially on the operating frequencies that are used; while the structures of AOFW elements for an operating frequency of, for example, 433 MHz typically have structural widths of ≧1 μm, the structural widths in the design for an operating frequency lie in the upper UHF range, for example at 2.4 GHz, at only roughly 300 nm. In combination with small film thicknesses, this produces small line cross-sections, which in succession calls for the use of materials with good electrical conductivity.
As a final criterion, the use of materials that (i) are economically available and (ii) can be processed with standard processes for thin-film deposits is advantageous for technical as well as commercial reasons.
In standard applications, the surface structures of AOFW elements preferably consist of aluminum, as explained in detail in EP-B1 0762641 or Buff et al., Proc. 2003 IEEE Ultrason. Symp., pp. 187-191. Al metal structures are light, readily electrically conductive, and able to be structured exactly. Thin aluminum films, despite a melting point at 660° C., however, are already soft starting from about 300° C. and susceptible in particular to voltage migration. Moreover, Al shows a high chemical affinity to oxygen and is oxidized by the latter to form electrically non-conductive aluminum oxides. While this oxidation is limited at temperatures of <400° C. to a surface film with a typically 5-10 nm thickness, and thus the conductivity of the metal structure is maintained, the oxide film grows at higher temperatures up to thicknesses in the micron range. With thin films, this can result, under certain circumstances, in a complete oxidation of the metal structure and thus a breakdown of the bonded element. Since the majority of high temperature-resistant piezoelectric substrates contain oxygen and the latter is at least partially chemically available at temperatures of several 100° C., this effect cannot be prevented in many cases even by a packaging in an oxygen-free atmosphere. Thus, as a whole, Al films do not represent any suitable solution for producing electrically conductive thin-film metal structures for high-temperature applications.
From the literature, a number of alternative materials are known for producing high temperature-resistant thin-film metal structures; a corresponding composition for AOFW elements is deduced, for example, from Hornsteiner et al., Proc. 1998 IEEE Freq. Control Symp., pp. 615-620 or U.S. Pat. No. B1 6,958,565. Frequently used materials in this case are in particular platinum and other metals of the platinum group of the periodic table.
Although Pt, starting from its melting point of 1769° C., is readily suitable as a material for high-temperature applications and is used in various applications, Pt films with film thicknesses of ≦1 μm are temperature-resistant only to a limited extent. In particular, Pt thin films that start from roughly 500° C. have a tendency toward dewetting and islanding, which in practice can result in a decomposition of structures even after a short period of application. In addition to the loss of electrical conductivity by destruction of the film continuity, the dewetting also reduces the coupling of the film to the substrate. Analogous effects can also be observed in other relevant metals, such as, for example, Pd or W, and also in alloys such as Pt+Rh or Pt+Au.
Relative to the tendency toward dewetting, it generally holds true that the effect is all the more greatly pronounced the thinner the film or the more narrow the structures are. Stable films are obtained only with film thicknesses and structural widths of, in each case, at least several μm, which runs contrary to the above-listed requirements of the surface structures of AOFW elements at several points. This problem is further intensified in practice in that materials that are relevant for high temperature-resistant metal structures consistently have a significantly higher density than aluminum. To keep the surface load and thus attenuation and coupling efficiency roughly constant, thin films are absolutely necessary here.
To avoid dewetting effects, first the use of adhesive films, for example that consist of Ti or Zr, which generally also act as diffusion barriers between substrate and adhesive film at the same time, is proposed in the technical literature (Hornsteiner et al., Phys. Status Solidi A, 163, pp. R3-R4; Buff et al., Proc. 2003 IEEE Ultrason. Symp., pp. 187-191; Thiele & da Cunha, Electron. Lett. 39(10), pp. 818-819; U.S. Pat. No. B1 7,285,894, etc.). Also, the use in most cases of oxidic, cover and passivation films, for example made of SiO2, or silicon-aluminum oxynitrides (SiAlON), was also proposed.
Thus, in practice, dewetting and other temperature-induced ageing effects can be reduced, but cannot be permanently reliably prevented. As remedies, films consisting of (i) high-melting metals, in particular Pt, Rh or Pt+Rh—, Pt+Ir— or Pt+Au alloys, and (ii) zirconium dioxide (ZrO2) were proposed in, for example, WO-A2 2009/035797, da Cunha et al., Proc. 2007 IEEE Ultrason. Symp., pp. 2107-2110, as well as various related publications.
Such metal/ZrO2 films with typical overall thicknesses of 100-150 nm are considered the technically best approach at this time for producing thin-film bondings for high-temperature applications. In this case, ZrO2 acts as the component stabilizing the metal film and thus preventing dewetting and islanding. As adhesive films and diffusion barriers, preferably thin Zr films are used; optionally, the film can be protected, moreover, with a passivation film, in particular made of SiAlON. Thus, the application of corresponding thin-film electrodes up to temperatures of 1000° C. is possible. Nevertheless, this solution, in practical use, has various drawbacks that significantly impair the usability and commercial competitiveness.
According to the literature, ZrO2 shows an extremely low tendency toward mixing with all commonly used high temperature-resistant conductive metals (cf. WO-A2 2009/035797 [0062] and [0095]). To produce a stable, electrically thoroughly conductive homogeneous film, it is thus necessary in such systems to build up the conductive film from a large number of very thin films, which are in contact with one another in a film-penetrating manner. With the deposits of higher film thicknesses, the oxide films would act as insulator films between the conductive films, which would impair the conductivity as well as the high-frequency properties of the films. Analogously to this, the danger of dewetting exists when using thicker conductive metal films—caused by the essential two-dimensional form of the oxidic support films.
In the previously known practice, this is achieved by alternating deposits of multiple, in each case narrowly-tolerated, thin (extremely thin) films (typical film thicknesses of 0.5-5 nm in each case) of the conductive metal and of Zr or ZrO2 or by means of Co deposits of conductive metal and Zr in an oxygen-containing reactive gas atmosphere. Both are connected with a high production cost.
A second relevant drawback is that Zr or ZrO2, as well as various conductive metals thus used in combination, e.g., Rh, are not included in the thin-film deposits of commonly used standard materials. Together with the stringent requirements for uniform, reproducible deposits of extremely thin films, high production costs result therefrom that stand in the way of widespread use in commercial applications of AOFW elements that are built up in such a way.
A third significant drawback arises from the typical material properties of the previously known film structures. Pt has a considerably poorer conductivity than, e.g., Al, which makes necessary correspondingly large conductor cross-sections for adhering to a reliable resistance value. Since the film thickness is limited by the considerable density of the Pt metal, this requires structural widths of typically ≧1 μm. Thus, Pt or Pt/ZrO2 structures are limited to operating frequencies of <1 GHz, which greatly limits the selection and measuring sensitivity of such systems.
The use of Rh as a conductive metal that is proposed as a remedy does improve the basic technical situation in comparison to Pt as a result of its lower density and better electrical conductivity; the fact that it is a practicable solution that is only for special applications in, e.g., the military field, is due, however, to the high production and material costs, in particular of Rh films.
As a whole, to date there thus does not exist any technically satisfactory solution for the production of high temperature-resistant, electrically readily conductive thin-film structures in general and UHF-suitable AOFW-metal structures in particular, which can be produced economically and thus are competitive with alternative technologies.